He certainly does not look it, but Eddie Redmayne has just turned 30. The reason I'm aware of this is that I was perusing the actor's fan site before I met him – a couple of weeks prior to the big day, 6 January – and there was anguished discussion among the online community about what to give him as a gift. They settled on a scrapbook containing short messages and possibly photographs – "but please no inappropriate pictures" – that the ringleaders, Ivonne and Kate, would present to him on his birthday when he would be performing on stage at the Donmar Warehouse in London.I don't want to spoil the surprise, I tell him towards the end of a relentlessly enjoyable two-hour lunch at a pub in Barnsbury, north London, but I thought you should be prepared. "Maybe I would want something inappropriate, that could be curious," he muses. "But you have to be careful what you say. I did an interview once where I was asked who I found attractive and I went on about cartoons and Nala from The Lion King – and it's a bit weird but various of my ex-girlfriends actually did look like Nala. Then it got to my birthday when I was doing a play called Red and I was walking down the stairs at the Donmar and there was this lovely girl and she said, 'Happy birthday!' and handed me a Nala toy. It's very sweet but at the same time, oh my God!"

Redmayne stops himself and laughs, "I can't believe I'm having a conversation about 'my fans'."

Suit and printed shirt by Burberry Prorsum. Photograph: Jason Hetherington for the Observer

These are golden days to be a Redmayniac, as those admirers are known. Made in Chelsea – he is the middle of three brothers, with an older half-brother and half-sister – he was educated at Eton and started attracting attention almost a decade ago when he took leave from university to play Viola in Mark Rylance's all-male Twelfth Night. After graduating from Cambridge, Redmayne favoured edgy, credible films and won an Olivier Award over here and a Tony in New York for his performance in Red, both in 2010, but it is in the past few months that it feels like he has definitively tipped.

The run started in November with My Week with Marilyn in which he starred, alongside Michelle Williams as Monroe and Kenneth Branagh's Laurence Olivier, as a callow runner who finds himself the confidant to the world's biggest film star. If you hurry, you might be able to catch him as the vulnerable, vacillating Richard II in Michael Grandage's mesmerising final production for the Donmar. Now – rounding off the trinity of film, theatre and TV – he is playing Stephen Wraysford in the BBC's breathlessly awaited adaptation of Birdsong, the Sebastian Faulks novel. He is also currently the face (and especially the cheekbones) of Burberry and recently appeared in US Vogue photographed in Ibiza by Mario Testino in various states of undress with the American supermodel Karlie Kloss.

"That's the one my brothers started getting angry about," he admits. "They were, 'All right, your life, stop it now.' But this bombardment will slow down come February. The world will be sick of my face."

Polka dot shirt and mohair jumper, both by Marni (mrporter.com). Photograph: Jason Hetherington for the Observer

As for turning 30, Redmayne is just hoping that it won't jinx a career that is going rather well right now. "An older actor said something to me many years ago: 'The thing about you Ed, is that you look young for your age, so you'll have quite a good run of it until you're 30 and then it'll be interesting to see what happens.' He didn't say it in a cruel way, but it's stuck in my mind.

"Fortunately, as a guy, you're luckier than the women, who have a really rough time of it. I have a lot of good actress girlfriends and when it gets to a movie level it's all about weight and producers sending you to boot camps and being really quite brusque with it. The guys get it, too: you have to buff up and there are gentle nudges that you're…" He trails off, then exclaims: "You didn't hire me for my pecs and, if you did, you fucked up! You hired me for my slightly off-kilter freckly appearance."

Redmayne is like this – funny, opinionated and self-deprecating to a degree rare among actors. He draws attention to his reddish hair (the reason, in his reading, that he was cast as Julianne Moore's son in the peculiar incest drama Savage Grace) and his generous, feminine lips (which he believes led to him having Angelina Jolie as a mother in Robert De Niro's The Good Shepherd).

"As an actor there's a lot of scrutiny and, even when you've had success, it becomes about sustaining that success," says Redmayne. "A friend of mine described it as a peakless mountain. Even for De Niro there's Pacino and for Pacino there's De Niro. You're never going to reach the top, which is why it's important to take time out and be content with my luck to be working in three mediums with wonderful people. Just to say, 'Right, good. It could be all downhill from here, but for now, it's lovely…'"

Ralph Fiennes and Sam Mendes were going to do it, but couldn't wrestle the material into feature-film dimensions. Other directors attached to Birdsong, since it was published in 1993, include Joe Wright, Paul Greengrass and Peter Weir. "If you've been a young British actor in the past 10 years, you'll have been up for it," says Redmayne, as darkness closes in outside and someone from the pub lights a fire behind us. "But none of those film versions ever worked because the love story has to be such a slow build between these people, as it is in the book – you can't do that in two hours of film."

Eventually the producers, Working Title, resolved to make Birdsong for television, over two 90-minute episodes. Screenwriter Abi Morgan, the currently ubiquitous force behind The Iron Lady and Shame, finally unlocked the script after nearly a decade of trying and TV stalwart Philip Martin was brought in to direct. French actress Clémence Poésy was cast as fatal woman Isabelle Azaire, while Redmayne found himself the right age at the right time and beat off strong competition to play Wraysford. "It feels like a perfect thing where the medium matches the material," he says.

Redmayne as Stephen Wraysford in Birdsong. Photograph: Giles Keytes/BBC/Working Title

On the evidence of Sunday's first episode, he's right – Birdsong is that rare thing: a screen adaptation of a book that is not a pale approximation. Redmayne is chilling in the lead role, while Morgan ingeniously weaves between the two main settings of the novel: genteel France in 1910 and the country, ravaged by war, in 1916. Previous versions of the script had followed Wraysford's life chronologically; now the focus is how this serious but passionate young man becomes such a superstitious, oddly masochistic soldier.

"It was very odd on set, like two different films," says Redmayne. "We shot the war stuff first on this huge field outside Budapest with the most astounding rabbit-warren trenches, and it was incredibly hot and intense. But it was in Budapest and you had all the boys, this amazing collection of British actors like Matthew Goode and Thomas Turgoose, and – of course, they had days on and days off – so they were out on the lash in Budapest. I felt like the depressingly boring dad, living last night's action vicariously through them.

"Then they all left after this hardcore six weeks, and these beautiful French actors arrived," he continues. "It became this European period drama, so make-up off, mud off and suddenly into these starched ties. Philip [Martin] and I were like, 'What's happening here?'"

Redmayne admits that 2011 was "quite a workaholic year" and that "I'm kind of spent, I'm knackered." But if his career is going to implode spectacularly in his 30s, he is determined to go out with a bang: after Richard II, he's straight into singing practice for his role as Marius in Tom Hooper's Les Misérables film. Redmayne is just hoping to find a couple of weeks between rehearsals and the shoot, so he can find a beach with white sand and transparent water. "My dad has an expression: 'Your thumb in your bum and your mind in neutral' and I want to do that somewhere," he says. "But probably after a day, the guilt would kick in. And besides, I'm pretty pale and moley, so I'll only sit in the sun and get more freckles."Fashion editor: Helen Seamons

Theo Bosanquet: Old Etonians such as Tom Hiddleston and Eddie Redmayne are just the half of it. The costs of entering the profession mean that theatre is in danger of becoming a playground for the privileged